Imaging (or radiography) with thermal neutron beams is an important way of studying man-made or natural materials and structures, such as fuel cells, batteries, car engines, cultural heritage objects, etc., on a wide range of scales from atomic through mesoscopic to macroscopic. The energy spectrum of the beam is determined by the properties of a neutron moderator, which produces either cold (1-10 meV) or thermal (10-80 meV) neutrons by moderating the high-energy neutrons produced by various methods. The collimation of the beam determines the spatial resolution of the imaging instrument.
A typical set up for neutron collimation is shown schematically on FIG. 1. To create a collimated beam, a small aperture 14 of the diameter, D, is located at a distance from the neutron source 12. The collimation is characterized by the L/D ratio, where L is the aperture-to-object distance. The object 16 is placed as close as possible to the neutron detector 18. The L/D ratio would normally span a range of 300-600, but can go as high as 6,000 for high-resolution imaging. To achieve such a high L/D for a reasonable L, the neutron aperture is limited to a small diameter, thus severely restricting the flux illuminating the object 16 (i.e., D is much smaller than a typical size of thermal neutron sources, which can be as large as 200 mm). Consequently, high-resolution neutron imaging typically requires high-flux research reactors, which are not easily accessible.
Moreover, and despite recent progress with neutron sources, neutron instruments remain limited, sometimes severely, by available neutron fluxes; and this limitation is particularly acute for compact accelerator-based neutron sources. Consequently, progress in neutron optics and instrumentation provides a path toward more-effective neutron instruments that is as important as the development of brighter sources.
Relatively weak interactions of neutrons with most materials give neutron radiation its penetrating power. However, the weakness of interactions results in the refractive index, which is very close to unity for most materials, 1−n≈10−6. Consequently, neutrons reflect from surfaces only at grazing angles, which are normally not larger than a few degrees. The refractive index depends on the square of the neutron wavelength so that refractive optics are strongly chromatic, which is a considerable disadvantage for instruments operating with polychromatic neutron beams. As a result, it is challenging to build efficient neutron optical components, such as lenses and mirrors normally used in visible-light optics. Currently, several kinds of neutron-focusing mirrors exist. Elliptical Kirkpatrick-Baez (KB) mirrors have been recently developed, following their successful use for synchrotron x-rays. KB mirrors can be precisely figured with low roughness and coated with multilayers having high a critical angle. For large neutron sources of 5-50 mm, however, such mirrors are not always optimal since they work best for sources of less than 1 mm. Furthermore, elliptical KB mirrors are not ideal as imaging devices, since the magnification of an elliptical mirror depends on an incident angle, leading to distortions in imaging of large objects. Somewhat similar toroidal mirrors are used in small-angle neutron scattering (SANS), but with limited success so far.